The main idea is that 3-phase and high-frequency generators deliver a much steadier, higher-quality power to the x-ray tube than a single-phase unit. Because the voltage waveform has very low ripple, the beam is more uniform from moment to moment, which means more consistent exposure and beam quality. This stability lets the tube operate at higher instantaneous current without the voltage sag that plagues single-phase systems, so you can achieve shorter exposure times or higher mA settings without overheating the tube. At the same time, the near-constant voltage makes x-ray production more efficient, so more of the input power becomes useful radiation rather than being wasted in fluctuations. Put together, these factors—the improved voltage stability, the greater available power, and the better heat-handling/efficiency—explain why all three advantages described apply to 3-phase or high-frequency generators.